1. Field of the Invention
This invention relates generally to the art of metal rolling, and more specifically to forming rolls for processing a strip of metal, generally referred to as the skelp, into welded tubes. Still more specifically, the invention deals with a cooperative pair of top and bottom rolls in a rolling mill for the initial breakdown step of the skelp.
2. Description of the Prior Art
The manufacture of welded tubes or pipes by rolling, which represents a major field of tube production today, is made possible by a variety of methods such as those known as edge forming, center forming, and circular forming. According to a well known train of shaping rolls adopted in such tube fabrication, a skelp is fed successively through a breakdown pass, fin pass, welding pass, and sizing pass. Another known roll train is such that the skelp travels through a succession of breakdown pass, cluster rolls, fin pass, welding pass, and sizing pass.
The breakdown pass has heretofore been defined by a concave bottom roll and a convex top roll (shown in FIG. 1 of the drawings attached hereto) for rolling the skelp into generally arcuate cross sectional shape. This conventional type of breakdown roll has difficulty in curling the opposite longitudinal edge portions of the skelp into arcs of a desired radius. The contours of the breakdown rolls are such that they fail to exert sufficient compressive forces on the edge portions of the skelp. Accordingly, after passing the breakdown rolls, the skelp tends to spring back. This makes it necessary for the subsequent fin pass rolls to apply greater forces to the work than would be required if it were not for such springback, with the consequent likelihood of ruining the rolls or the fins. The springback of the skelp edges also makes difficult the welding of the edges and, at the sizing pass, the processing of the welded tubing into exactly circular cross sectional shape.
An additional drawback of the conventional pair of breakdown rolls is the high possibility of the skelp developing wavy edges, particularly if it is thin. Moreover, in the concave bottom roll of the prior art breakdown roll pair, the roll surface rises steeply as it extends from the midpoint of the roll, where it is of minimum diameter, toward its opposite axial ends. Thus, a considerable difference exists between the peripheral speeds of the center and the end portions of the concave roll. The resulting slip of the roll end portions over the skelp often produces the so-called roll marks thereon.